PA28 instructor and student inadvertently turn onto a restricted taxiway; stating signage was not visible.

Narrative

Flew to a nearby airport; BRD. Two runways where closed at the BRD airport due to a resurfacing project. We therefore needed to depart on a runway that we seldom used. While taxing to the active runway we inadvertently turned onto a restricted taxiway that led to a Department of Natural Resources fire base. This taxiway angled off of the taxiway we were on at more than a 90 degree angle from our taxi direction and appeared to provide a direct route to the departure end of the active runway. After realizing that we were heading towards the fire base ramp; we turned around and went back to the taxiway we had been on. After turning back onto the taxiway we had been on; we could now see that there was a sign indicating that the other taxiway was restricted but this sign was not visible from the direction that we had been taxing because of the angle of the taxiway. The restricted sign appeared to be temporary (used only during fire season?) and we speculated that a sign that should have been visible from the direction we had been taxing could have fallen down or simply not been put up. We also noted that two sets of taxiway diagrams we had for the airport; commercial IFR charts and the state of Minnesota airport directory; did not show that the taxiways or ramp near the fire base were restricted.